Active Mechanics in Living Oocytes Reveal Molecular-Scale Force Kinetics

Active diffusion of intracellular components is emerging as an important process in cell biology. This process is mediated by complex assemblies of molecular motors and cytoskeletal filaments that drive force generation in the cytoplasm and facilitate enhanced motion. The kinetics of molecular motors have been precisely characterized in-vitro by single molecule approaches, however, their in-vivo behavior has remained elusive. Here, we study the myosin-V driven active diffusion of vesicles in mouse oocytes, where this process plays a key role in nuclear positioning during development, and combine an experimental and theoretical framework to extract molecular-scale force kinetics in-vivo (motor force, power-stroke, and velocity). We find that myosin-V induces rapid kicks of duration τ∼300 μs resulting in an average force of F∼0.4 pN on vesicles. Our results reveal that measuring in-vivo active fluctuations allows extraction of the underlying molecular motor activity and demonstrates a widely applicable mesoscopic framework to access molecular-scale force kinetics.